Anti-slip control

ABSTRACT

A method for preventing the driven wheels of an automotive vehicle from spinning provides that whenever one of two driven wheels of one axle of the vehicle has a lower friction coefficient with the road surface than the other wheel of said axle and the one wheel shows a spinning tendency, brake pressure is built up at said wheel, with the rotational speed of the other wheel being measured, and whenever the rotational speed exceeds a specified value with respect to a vehicle reference speed, the brake pressure at the one wheel is reduced or maintained constant.

The invention relates to the so-called anti-slip control (ASC), i.e. toa method for preventing the driven wheels of a vehicle from spinning.

Such ASC systems are also referred to as TC (traction control) systems.

The driven wheels of a vehicle always have a tendency to spin when thevehicle engine generated driving torque at the wheel is higher than thetorque based on the momentary friction coefficient that can betransmitted via the wheels to the road.

Similar to block-protected braking (ABS), not only the driving torquethat can be transmitted from the wheel to the road surface in thedriving direction decreases with increasing traction slip afterexceeding a maximum value, but also the cornering force of the wheelsdecreases to very low values after exceeding a maximum.

From the state of the art traction slip controllers are known, where onthe one hand spinning of the driven wheels is prevented by reducing thedrive power of the engine in the case of a spinning tendency of a wheel.On the other hand, traction slip controllers are known, where in thecase of a spinning tendency of a driven wheel, the brake of that wheelis automatically actuated in order to improve the traction of thatwheel. The state of the art generally provides for controlling bothdriven wheels of one axle on the basis of their individual rotationbehaviour with respect to traction.

If, for example, one wheel of the axle shows a spinning tendencysignificantly before the other wheel (because the one wheel runs on asmoother road surface), the brake of the wheel that is the first to showa spinning tendency is supplied with brake pressure. When using adifferential gear, the consequence for the respective axle is that theother driven wheel (not yet showing a spinning tendency) transmits ahigher driving torque to the road surface.

With this known traction control system, the wheel which has firstbecome instable (i.e. the wheel which is the first to show a spinningtendency) is being braked as long as is required by the rotational speedof the wheel. In other words: as long as the rotational speed of thewheel is exceeding a given threshold value, it is braked in order toprevent spinning.

If the other wheel which at first was stable also shows a spinningtendency, then this wheel is also braked accordingly.

DE 40 22 471 A1 discloses a traction control system for increasing thedriving stability, where the driven wheel which momentarily shows ahigher friction coefficient is determined, and the rotational speed ofthis wheel is monitored and stabilized by a braking effect. A brakepressure prevailing in the wheel brake of the second driven wheel of thesame vehicle axle will be relieved. The control is therefore completelybased on the rotational behaviour of the stable wheel, the brakepressure of which is controlled as a function of its rotational speed.

The present invention is based on the objective to provide a(convenient) anti-slip control system which annoys the driver as littleas possible, which can function with a simple control algorithm, andwhich imposes only minimum load on the vehicle components.

First, the invention is based on the following finding:

With a conventional traction control system, in particular with rapidlychanging friction coefficients between the wheel and the road surface,large variations of the braking torques prevailing at both wheels of oneaxle occur, and this leads to an extreme load acting on the gears,bearings and transmissions, particularly in the case of a vehicle axlewith a so-called differential. This highly impairs the durability andthe life of the drive train of the vehicle.

With conventional traction control systems, such loading of thecomponents occurred particularly whenever both wheels of a drive axleare coupled via a differential. In the case of one wheel of the axlecoming into the instable range, i.e. showing a spinning tendency, itwill be braked down by the actuation of its wheel brake to a lowerrotational speed. Because of the differential, this again results in theother wheel, which has a higher friction coefficient and which istherefore still running stably, generating a higher drive torque andtherefore being at risk to spin itself. This spinning tendency might beso strong that the cornering stability is affected significantly. Ifthis second wheel which has become instable at a later time is alsobraked, the above mentioned high loads result, thus causing wear of thebraking system, the clutch and the transmission.

The above mentioned objective is achieved by the invention with ananti-slip control system for a two-track automotive vehicle in such amanner that whenever one of two driven wheels of one axle of the vehiclehas a lower friction coefficient with the road surface than the otherwheel of said axle and the one wheel shows a spinning tendency, a brakepressure is built up at this wheel while the rotational speed of theother wheel is measured, and, with the rotational speed exceeding aspecified value with respect to a vehicle reference speed, the brakepressure at the one wheel is relieved or kept constant.

A main characteristic of the anti-slip control system according to theinvention therefore is that whenever a significant difference in thefriction coefficients of the two driven wheels of one axle occurs, thebrake pressure at only one of the two wheels, namely that wheel which isthe first to become instable, is essentially increased automatically,with the rotational behaviour of the still stable wheel being measuredand the brake pressure in the wheel brake of the firstly instable wheelbeing controlled as a function of the rotational speed of the stillstable wheel over the course of time. The overall control is thereforeessentially reduced to a single wheel, namely to the wheel which is thefirst to become instable which runs on a smoother road surface, wherethe essential parameters for the control of the brake pressure in thiswheel which is the first to become instable are the rotationalproperties of the still stable wheel.

According to the invention it is not attempted, as has been the generalpractice with the state of the art, to prevent the wheel that is thefirst to become instable from spinning, but rather the invention isbased on the idea to maintain the wheel running on a good road surface(with a high friction coefficient) within the optimum range of the slipcurve and, thus, ensure the maximum traction and the highest possiblecornering force.

The invention can advantageously be applied both to two and to fourwheel driven vehicles, in the latter case particularly when there is norigid drive-through engaged. In this case, a relief of the centredifferential lock in particular can be noted.

In the following, an embodiment of the invention will be described moredetailed with reference to the drawing in which:

FIG. 1 shows the rotational speed vs. time of the two driven wheels ofthe same axle of a vehicle, and the so-called vehicle reference speedvs. time, as it is known from the ABS and ASC technology;

FIG. 2 shows the brake pressure vs. time in the wheel brake of a firstwheel, the rotational speed of which is indicated in FIG. 1 as v₁ ; and

FIG. 3 shows the brake pressure vs. time in the wheel brake of a secondwheel of the same axle of the vehicle, the rotational speed of which isindicated in FIG. 1 as v₂.

As can be seen in the drawing, FIGS. 1, 2 and 3 have the same time axist, respectively.

FIG. 1 shows a typical curve of the rotational speed of the two drivenwheels of a drive axle of a vehicle during a starting process on aninhomogenous road surface. It is assumed that one wheel of the axle runson a relatively smooth road surface, namely that wheel the rotationalspeed of which is indicated as v₁ in FIG. 1, while the other wheel runson a relatively non-skidding road surface; its rotational speed isindicated as v₂ in FIG. 1.

The ASC control algorithm according to the invention is only appliedwhen a distinct difference in the friction coefficients of the twodriven wheels has been detected. Such a difference in the frictioncoefficients can be detected, for example, by the fact that over a givenperiod of time the rotational speed of the one wheel increasesconsiderably more than the rotational speed of the other driven wheel ofthe same vehicle axle. See also FIG. 1.

The curves of the rotational speeds v₁ and v₂ according to FIG. 1 arebased on the assumption that the attempt to start begins at time t₀,i.e. the driver engages the clutch and accelerates. The rotational speedv₁ of the first wheel increases steeply, while the rotational speed v₂of the second wheel (broken line) increases less steeply and, at a timet₂ drops below a threshold value v⁺ _(ref). According to FIG. 1, thethreshold value v⁺ _(ref) differs from the vehicle reference speedv_(ref) which is known as such by a constant value. The fact shown inFIG. 1 that at time t₁ the rotational speeds v₁ and v₂ of the two drivenwheels differ by more than a specified value, or, respectively, thecondition that at a high rotational speed v₁ of the first wheel therotational speed v₂ of the second wheel drops below the threshold valuev⁺ _(ref), indicate that the one wheel (v₁) runs on a considerablysmoother road surface than the other wheel (v₂). This is why the ASCcontrol algorithm described in the following is applied.

Thus, at time t₁ the rotational speed v₁ of the first wheel increasesfurther while the rotational speed v₂ of the second wheel drops belowthe threshold value v⁺ _(ref). The latter fact indicates that the secondwheel (v₂) runs stably.

According to FIG. 2, the brake pressure p₁ in the wheel brake of thefirst wheel is increased at time t₁. As can be seen from FIG. 3, thebrake pressure p₂ in the wheel brake of the second wheel (v₂) is notincreased over the entire time span between t₁ and t₇.

The course of the rotational speed v₂ of the second wheel (broken linein FIG. 1), however, is closely monitored, and the brake pressure p₁ inthe wheel brake of the first wheel is controlled according to FIG. 2 asa function of the rotational speed v₂ with respect to the thresholdvalue v⁺ _(ref).

Upon braking of the wheel which is the first to spin (v₁) that occursfollowing time t₁, the counter force is increased on that side of theaxle differential where the first wheel is arranged, and therefore ahigher drive torque is transmitted to the second wheel which is arrangedat the other side of the axle differential, i.e. the second wheelreaches an optimum range of the friction coefficient/slip curve.

According to FIG. 2, the brake pressure p₁ in the wheel brake of thefirst wheel is increased to a maximum value and then maintained constantuntil time t₂. At time t₂ the rotational speed v₂ of the second wheelhas reached (exceeded) the threshold value v⁺ _(ref) again, andtherefore, following time t₂, the brake pressure p₁ is reduced accordingto FIG. 2 and, upon reaching a specified value, maintained constantuntil a time t₃. At time t₃, the rotational speed v₂ of the second wheeldecreases below the threshold value v⁺ _(ref) again, and similar to theabove described process, the brake pressure p₁ in the wheel brake of thefirst wheel is increased again.

At time t₄, the rotational speed v₂ of the second wheel exceeds thethreshold value v⁺ _(ref) again, and the brake pressure p₁ in the wheelbrake of the first wheel is correspondingly reduced again and thenmaintained constant (see also FIG. 2). With each reduction of the brakepressure p₁ in the brake of the first wheel, the torque which the driveapplies to the second wheel because of the differential gear is alsoreduced, so that the rotational speed of the second wheel again reachesthe optimum range about the threshold value v⁺ _(ref) (see FIG. 1).

Although such a reduction of the brake pressure p₁ causes more spinningof the first wheel during the time interval from t₄ to t₅, however, asexplained above, the second wheel always returns into the optimum sliprange. Under the described conditions, the second wheel is not braked atall, see also FIG. 3, time span t₀ to t₇, during which no brake pressurep2 is supplied to the wheel brake of the second wheel.

FIGS. 1, 2 and 3 also illustrate the case of abruptly changing roadconditions, in that the first wheel (v₁) which was initially running ona very smooth road surface, reaches a very non-skidding road surface attime t₅, while at the immediately following time t₆ the second wheel,which was initially running on a very non-skidding road surface, reachesa very smooth road surface. Such a "checkered" friction coefficientchange between the wheels constitutes the most difficult extreme casefor an anti-slip control system.

As shown in FIG. 1, the rotational speed v₁ of the first wheel dropsconsiderably after time t₅, while the rotational speed v₂ of the secondwheel increases considerably after time t₆. This can be explaineddirectly from the previously assumed friction coefficient relations.From the abrupt inverse change of the rotational speeds v₁ and v₂, thecomputer of the ASC system detects that the friction coefficientsbetween the wheels and the road surface have changed dramatically. Whensuch a situation prevails, the brake pressure must be transferred asquickly as possible from the one wheel into the wheel brake of thesecond wheel, as can be seen from comparing FIGS. 2 and 3. Such anextreme, checkered change of the friction coefficients from one wheel tothe other is the only situation where it is appropriate for the controlalgorithm according to the invention to accept pressure in the brakes ofboth wheels, which is the case during the time span between t₇ and t₈,as shown in FIGS. 2 and 3. The simultaneous pressure reduction in thefirst wheel and the pressure build-up in the second wheel as intendedhere are advantageous in that they enable a smooth transition and aminimum acceleration drop. Under the aspect of preserving the materials(life of the components), too, the short time span from t₇ to t₈ withsimultaneous braking on both sides is not detrimental, because in thepreviously braked first wheel only a very small decreasing pressureexists, while during this time span the pressure built up in the secondwheel has not reached very high values. Following time t₇, the secondwheel will now be the instable one, contrary to the above description,so that the brake pressure p₂ in the wheel brake of the second wheelwill be controlled according to FIG. 3 depending on the relation ofrotational speed v₁ of the first wheel to the threshold value v⁺ _(ref).The control is effected completely analogously as described above, onlywith the wheels exchanged.

We claim:
 1. A method for preventing the driven wheels of an automotivevehicle from spinning, where whenever a first wheel of two driven wheelsof an axle of the vehicle provided with an axle differential has a lowerfriction coefficient with the road surface than a second wheel of thataxle, and the first wheel shows a spinning tendency, but not the secondwheel, a brake pressure is built up at the first wheel, and then therotational behavior of the second wheel which does not show a spinningtendency is measured and evaluated so that whenever the rotational speedof the second wheel exceeds a threshold value (v⁺ ref), the brakepressure at the first wheel is temporarily relieved, and whenever therotational speed of the second wheel drops below the threshold value (v⁺ref), the brake pressure at the first wheel is temporarily built up. 2.The method of claim 1 wherein upon an eventual increase of the spinningtendency of the second wheel, with a corresponding reduction in thespinning tendency of the first wheel to a quantity lower than that ofthe second wheel, the rotational behavior of the first wheel which doesnot show a spinning tendency is measured and evaluated so that wheneverthe rotational speed of the first wheel exceeds a threshold value (v⁺ref) the brake pressure at the second wheel is temporarily relieved andwhenever the rotational speed of the first wheel drops below thethreshold value (v⁺ ref) the brake pressure at the second wheel istemporarily built up, thereby permitting a transitional time periodduring which braking of the first and second wheels occurssimultaneously, the transitional time period commencing when therotational speed of the second wheel becomes equal to the rotationalspeed of the first wheel.